US 5483962 A Abstract Entered complex data is phase-rotated by the corresponding to the moving velocity of the clutter component. Then clutter component information is eliminated by a real type MTI filter assembly. Thereafter, the output data of the real type MTI filter assembly is reversely phase-rotated.
Claims(16) 1. An ultrasonic diagnostic system with a complex MTI filter, comprising:
probe means for detecting ultrasonic information and outputting a probe signal; complex MTI filter means for receiving said probe signal including first phase rotating means for receiving angle data and complex data, which are associated with each other, to rotate a phase of a complex number represented by the received complex data by an angle indicated by the associated angle data; first and second real type MTI filters for receiving data representative of the real part and the imaginary part of the complex number, which have been subjected to the phase rotation by said first phase rotating means, to provide predetermined filtering processes for said data representative of the real part and the imaginary part of the complex number, respectively; second phase rotating means for receiving angle data and complex data of which real part and imaginary part are given with said data subjected to the predetermined filtering processes by said first and second real type MTI filters, respectively, which angle data and complex data are associated with each other, to rotate a phase of a complex number represented by the received complex data by an angle which is the same as the angle indicated by the associated angle data in the absolute value but is of inverse sign, so that an output complex data is generated; and display means for displaying information based upon the probe signal as filtered. 2. An ultrasonic diagnostic system according to claim 1, wherein said complex MTI filter means further comprises angle accumulating means for receiving angle data to sequentially accumulate angles itself indicated by said angle data so as to obtain accumulated angles, and cosine and sine calculating means for obtaining data representative of cosine and sine of said accumulated angles obtained by said angle accumulating means and supplying the thus obtained data in the form of angle data to said first and second phase rotating means.
3. An ultrasonic diagnostic system according to claim 1, wherein said first phase rotating means is provided with complex multiplication means for performing a complex multiplication of a complex number represented by the complex data entered said first phase rotating means by an additional complex number consisting of the real part and the imaginary part which are given by cosine and sine of an angle represented by the angle data entered said first phase rotating means, respectively.
4. An ultrasonic diagnostic system according to claim 1, wherein said second phase rotating means is provided with complex multiplication means for performing a complex multiplication of a complex number represented by the complex data entered said second phase rotating means by an additional complex number consisting of the real part and the imaginary part which are given by cosine and sine of an angle represented by the angle data entered said second phase rotating means, respectively.
5. An ultrasonic diagnostic system according to claim 1, wherein said complex MTI filter means further comprises factor selecting means for selecting filter factors of said first and second real type MTI filters.
6. An ultrasonic diagnostic system according to claim 1, wherein said first and second real type MTI filters each are a filter of any of moving average type, autoregressive type and autoregressive moving average type.
7. An ultrasonic diagnostic system according to claim 1, further comprising an additional filter provided after said second phase rotating means for eliminating a DC component of the complex data output from said second phase rotating means.
8. An ultrasonic diagnostic system according to claim 1, wherein said angle data is indicative of an angle corresponding to the movement of the clutter within the human body under examination, which movement being measured by utilizing an ultrasonic Doppler effect.
9. A diagnostic system with a complex MTI filter, comprising:
a beamformer for forming, based upon a probe signal, a received signal carrying ultrasonic reflection information as to each point on a scan line extending inside the subject; a quadrature detector for practicing a quadrature detecting treatment to the received signal formed by said beamformer to generate a complex signal; an angle arithmetic unit for evaluating angle data on the basis of the complex signal generated by said quadrature; detector; a complex MTI filter for receiving the complex signal derived from said quadrature detector and the angle data derived from said angle arithmetic unit, including first phase rotating means for receiving angle data and complex data, which are associated with each other, to rotate a phase of a complex number represented by the received complex data by an angle indicated by the associated angle data; first and second real type MTI filters for receiving data representative of the real part and the imaginary part of the complex number, which have been subjected to the phase rotation by said first phase rotating means, to provide predetermined filtering processes for said data representative of the real part and the imaginary part of the complex number, respectively; second phase rotating means for receiving angle data and complex data of which real part and imaginary part are given with said data subjected to the predetermined filtering processes by said first and second real type MTI filters, respectively, which angle data and complex data are associated with each other, to rotate a phase of a complex number represented by the received complex data by an angle which is the same as the angle indicated by the associated angle data in the absolute value but is of inverse sign, so that an output complex data is generated; blood flow information arithmetic means for obtaining blood flow information within the subject based on the complex signal subjected to filtering in said complex MTI filter; and display means for displaying the blood flow information obtained in said blood flow information arithmetic means. 10. A diagnostic system according to claim 9, wherein said complex MTI filter means further comprises angle accumulating means for receiving angle data to sequentially accumulate angles itself indicated by said angle data so as to obtain accumulated angles, and cosine and sine calculating means for obtaining data representative of cosine and sine of said accumulated angles obtained by said angle accumulating means and supplying the thus obtained data in the form of angle data to said first and second phase rotating means.
11. A diagnostic system according to claim 9, wherein said first phase rotating means is provided with complex multiplication means for performing a complex multiplication of a complex number represented by the complex data entered said first phase rotating means by an additional complex number consisting of the real part and the imaginary part which are given by cosine and sine of an angle represented by the angle data entered said first phase rotating means, respectively.
12. A diagnostic system according to claim 9, wherein said second phase rotating means is provided with complex multiplication means for performing a complex multiplication of a complex number represented by the complex data entered said second phase rotating means by an additional complex number consisting of the real part and the imaginary part which are given by cosine and sine of an angle represented by the angle data entered said second phase rotating means, respectively.
13. A diagnostic system according to claim 9, wherein said complex MTI filter means further comprises factor selecting means for selecting filter factors of said first and second real type MTI filters.
14. A diagnostic system according to claim 9, wherein said first and second real type MTI filters each are a filter of any of moving average type, autoregressive type and autoregressive moving average type.
15. A diagnostic system according to claim 9, further comprising an additional filter provided after said second phase rotating means for eliminating a DC component of the complex data output from said second phase rotating means.
16. A diagnostic system according to claim 9, wherein said angle data is indicative of an angle corresponding to the movement of the clutter within the human body under examination, which movement being measured by utilizing an ultrasonic Doppler effect.
Description 1. Field of the Invention The present invention relates to a complex MTI filter, which when obtaining blood flow information within the human body under examination utilizing an ultrasonic Doppler effect, is adapted to separate the blood flow component information from clutter component information. 2. Related Background Art There has been used an ultrasonic diagnostic system for transmitting ultrasonic beams within the human body and receiving the ultrasounds reflected by a tissue in the human body thus diagnosing diseases of the viscera and the like of the human body. In one aspect of this ultrasonic diagnostic system, or in an optional function of an ultrasonic diagnostic system for displaying a tomographic image (B-mode), there has been used an ultrasonic Doppler diagnostic system in which ultrasounds reflected by blood cells flowing within the human body are received to obtain blood flow information such as velocity, variance, power and the like of the blood flow. FIG. 12 is a schematic construction view of one example of a conventional ultrasonic diagnostic system. A transmission control section 11 supplies pulse signals Tp to a number of transducers (not shown) constituting an ultrasonic probe 12 in each specified timing, and thus each transducer transmits ultrasonic pulse beams within the human body under examination (not illustrated). For example, in sector scanning, a specified number (e.g. eight pulses) of ultrasonic pulse beams are emitted along a given direction. The ultrasonic pulse beams are reflected by blood cells flowing within the human body under examination or the other tissues, and are received by each transducer in the ultrasonic probe 12. The received signals Ap received by each transducer are input to a beamformer 13 and is beamformed therein so as to reception dynamic focussing. The received signal S thus obtained is input in a B-mode image detecting section 14 and a blood flow information detecting section 15. The B-mode image detecting section 14 generates a signal S Meanwhile, the blood flow information detecting section 15 detects blood flow information on the basis of the input received signal S utilizing a Doppler effect as described below. Namely, the ultrasounds reflected by blood cells within the blood flow are subjected to frequency shift by movement of the blood cells. The frequency shift amount (Doppler shift frequency) f
f where V Further, the center frequency f of the received signal receiving the reflected ultrasounds is represented by as follows:
f=f Accordingly, it is possible to detect the blood flow velocity V The signal S Hereinafter, there will be described only detection of the blood flow information which is the subject matter of the present invention. FIG. 13 is a block diagram of one example of a portion equivalent to a blood flow information detecting section 15 as shown in FIG. 12 which is involved in the conventional ultrasonic diagnostic system. As shown, the received signal S output from the beamformer is input to a quadrature detector 17 to be detected by 90 degrees. FIG. 14 is a block diagram showing an internal construction of the quadrature detector 17. The received signal S input to the quadrature detector 17 is divided into two lines, which are respectively input to multipliers 171 and 172. Meanwhile, the multipliers 171 and 172 receive two sinusoidal signals (carrier signals) cos 2 πνt and -sin 2 πνt, which are different in phase by 90 degrees from each other. The multipliers 171 and 172 multiply the received signal S by the carrier signal: cos 2 πνt, and the received signal S by the=carrier signal: -sin 2 πνt, respectively, thus generating two signals each having both frequencies of addition and difference of the two signals prior to multiplication. These signals are made to pass through low-pass filters 173 and 174, respectively. This generates an I component and Q component of the received signal S after the quadrature detection, each carrying only signal having the frequency of difference between the two signals mentioned above. The I component and Q component of the received signal S, which have been subjected to the quadrature detection, are input into A/D converters 175 and 176 to be A/D converted, respectively, and are then temporarily stored in RAM's (Random Access Memory) 177 and 178. Thereafter, the I component and Q component of the received signal S after the quadrature detection are read out from the RAM's 177 and 178, and are then input to Moving Target Indicators (MTI) filters 18 and 19 shown in FIG. 13, respectively. The order of read out from the RAM's 177 and 178 is different from the order of load onto the RAM's 177 and 178. For example, the I components and Q components of the eight received signals S on each observation point within the human body under examination, which are obtained when ultrasonic beams are emitted eight times along a given direction within the human body under examination, are read out on each observation point from the RAM's 177 and 178, respectively, in accordance with the order of transmission of the ultrasonic beams. Now, it is assumed that a certain one point within the human body under examination is selected as a representation point, and signals read out from the RAM's 177 and 178 on the representation point are denoted by I Each of the MTI filters 18 and 19 to which the signals I The MTI filters 18 and 19 are involved in the subject matter of the present invention, and thus they will be described in detail later. Clutter removed signals BI In this manner, it is possible, on each observation point within the human body under examination, to obtain a blood flow signal S FIGS. 15(A) and 15(B) are views each showing the characteristic of the MTI filter. The axis of abscissa represents a Doppler shift frequency f As shown in FIG. 15(A), in a case where the blood flow velocity is high and the crest 33 is greatly separated from the crest 32, the clutter removed signals BI FIG. 16 is a block diagram showing a signal processing circuit corresponding to the blood flow information detecting section 15, which is so arranged that even in a case where the Doppler shift frequency of the clutter component comes closer to that of the blood flow component, the clutter component information can be selectively removed; and FIG. 17 is a view showing relationship between characteristic and received signal of the complex filter shown in FIG. 16. In these figures, the elements corresponding to those of FIGS. 13-15 are denoted by the same reference numbers or symbols as those of FIGS. 13-15, and the explanation thereof is omitted. A complex MTI filter 25 has a disadvantage such that the number of elements constituting the filter is increased thereby enlarging the magnitude of the circuit compared with the common real type MTI filter as shown in FIGS. 13 and 15. On the other hand, it has an advantage such that the center frequency of the signal elimination band is determined into the value other than zero. The signal processing circuit shown in FIG. 16 is made using the above advantage of the complex filter. An I component I Further, the I component I Incidentally, the variance σ In some determinations of the center frequency f As described above, the complex MTI filter permits the center frequency f Hereinafter, first, the conventional common real type MTI filter will be explained, and then the complex MTI filter constituted by simply combining the real type MTI filters will be explained. FIG. 18 is a circuit block diagram of a moving average type (MA type; Finite Inpulse Response (FIR) type) MTI filter, as an example of the conventional real type MTI filter shown in FIG. 13. The moving average type MTI filters 18 and 19 comprise multipliers 181 The conventional typical ultrasonic diagnostic system uses a four-order (four pieces of multipliers for multiplying input signal by factors are used) moving average type of MTI filter. However, nowaday clinic requires to detect also the blood flow in such a low velocity that it is close to the moving velocity of the clutter component. In order to satisfy this requirement, there is a need to provide a fine filter characteristic, and it is preferable to use an eight-order moving average type of MTI filter as shown in FIG. 18. If the moving average type of MTI filter as shown in FIG. 18 is used and then the factors K0, K1, . . . , K7 are suitably selected, as seen in FIGS. 15(A) and (B) it is possible to eliminate the signals within a frequency band of |f FIG. 19 is a circuit block diagram of an autoregressive type (AR type; Infinite Inpulse Response (IIR) type) MTI filter, as another example of the conventional real type MTI filter. The autoregressive type MTI filter does not need a large scale of circuit in extent of the moving average type MTI filter. However, according to this type of filter, the response is varied in accordance with initial values of the input signals I FIG. 20 is a circuit block diagram of a complex MTI filter comprising the real type of moving average type MTI filter. Assuming that two input signals I and Q are expressed by a complex number I+jQ consisting of a real part and an imaginary part (j denotes imaginary number unit); a complex factor K The complex filter shown in FIG. 20 is arranged with the real type MTI filter patterned after the equation (4). The complex MTI filter shown in FIG. 20 is over twice as large as the circuit scale of the real type MTI filter shown in FIG. 18. According to such a complex MTI filter, in order to selectively remove the clutter component information, it is necessary to interchange 16 pieces of factors K FIG. 21 is a circuit block diagram of a complex MTI filter comprising the real type of autoregressive type MTI filter as shown in FIG. 19. The complex MTI filter shown in FIG. 21 is also over twice as large as the circuit scale of the real type MTI filter shown in FIG. 19. Regarding a technology of obtaining the blood flow information upon separating it from the clutter component information, U.S. Pat. No. 5,170,792 discloses another example of the prior art. According to the scheme disclosed in U.S. Pat. No. 5,170,792, a signal output from a Doppler system processor system, which corresponds to the quadrature detector 17 shown in FIG. 13 of the present application, is phase-rotated by the corresponding velocity of the clutter component, and the signal subjected to the phase rotation is input to a MTI filter so that the clutter component information is eliminated. According to this scheme, there will be observed a blood flow velocity relative to the clutter component, which is obtained by means of subtracting a clutter component velocity to the ultrasonic probe 12 from a blood flow velocity to the ultrasonic probe 12. This is a useful scheme in such a requirement that a blood flow within internal organs is observed cancelling movement or performance of the internal organ itself, but it is not a general-purpose scheme. For instance, as in case of the heart, in a case where the clutter and the blood move independently of each other, it would be difficult to observe a proper blood flow velocity with respect to the heart of interest. Further, the scheme disclosed in U.S. Pat. No. 5,170,792 has been associated with an additional big problem which will occur when the apparatus is assembled. Specifically, in the quadrature detector 17 shown in FIG. 13 of the present application and the corresponding Doppler system processor system referenced to in U.S. Pat. No. 5,170,792, a DC offset emanates. It will be a cause of the superposition of the DC component on signals. Further, there is a possibility of the superposition of the DC component on signals due to various causes such as the ultrasonic component reflected by a stationary object. In order to obtain an exact blood flow velocity, however, it is necessary to completely remove those DC components. According to U.S. Pat. No. 5,170,792, a stationary bias signal canceller is used to remove the DC components. However, the stationary bias signal canceller is placed at upper stream stage than the tissue velocity estimation/compensation and the MTI filter with respect to a signal flow. Thus, there is a possibility such that the DC component is superposed on an input signal of the velocity estimator for obtaining the blood flow velocity owing to the bit error (occurrence of error caused by the fact that the numbers less than the Least Significant Digit can not be given) in the digital arithmetic operation in the mid course on the path in which a signal output from the stationary bias signal canceller reaches the velocity estimator for obtaining the blood flow velocity, and owing to the transient response in the MTI filter and the like. In this case, it would be difficult to detect a blood flow signal in a critical point with respect to the noise level and thus the apparatus would be limited in accuracy of the detection. Incidentally, according to the scheme of U.S. Pat. No. 5,170,792, the stationary bias signal canceller is permitted only to be placed at upper stream stage than the tissue velocity estimation/compensation with respect to a signal flow, and is not permitted to be placed immediately before the velocity estimator for obtaining the blood flow velocity. In view of the foregoing, it is therefore an object of the present invention to provide a complex MTI filter having a simple structure and capable of being implemented with the low cost in manufacturing. It is another object of the present invention to provide a complex MTI filter, when which is applied to an ultrasonic diagnostic system, capable of obtaining an absoute blood flow velocity to an ultrasonic probe, but not a relative blood flow velocity to a clutter component velocity. It is still another object of the present invention to provide a complex MTI filter, when which is applied to an ultrasonic diagnostic system, permitting a DC elimination filter to be placed lower stream stage than the complex MTI filter with respect to a signal flow. FIG. 1 is a block diagram showing the principle of the complex MTI filter according to the present invention. Now it is assumed that eight complex data representing the complex number I The complex MTI filter according to the present invention basically comprises first phase rotating means 1, first and second real type MTI filters 2 and 3, and second phase rotating means 4. The first phase rotating means 1 receives angle data and complex data, which are associated with each other, and rotates a phase of a complex number I The first phase rotating means 1 may be provided with complex multiplication means for performing a complex multiplication of a complex number represented by the complex data entered said first phase rotating means 1 by an additional complex number consisting of the real part and the imaginary part which are given by cosine and sine of an angle represented by the angle data entered said first phase rotating means, respectively. The first and second real type MTI filters 2 and 3 receive data representative of the real part and the imaginary part of the complex number, which have been subjected to the phase rotation by said first phase rotating means 1. These real type MTI filters are not restricted in the type. That is, each may be a filter of any of moving average type, autoregressive type and autoregressive moving average type which is a combination of the moving average type and the autoregressive type. Further, it is preferable that the real type MTI filters 2 and 3 include factor selecting means for selecting filter factors of said first and second real type MTI filters 2 and 3, respectively. The real type MTI filters 2 and 3 provide predetermined filtering processes for the data representative of the real part and the imaginary part of the complex number, respectively, in accordance with the filter factors. The second phase rotating means 4 receives angle data and complex data of which real part and imaginary part are given with said data subjected to the predetermined filtering processes by said first and second real type MTI filters 2 and 3, respectively, which angle data and complex data are associated with each other. The second phase rotating means 4 rotate a phase of a complex number represented by the received complex data by an angle which is the same as the angle θ FIG. 2 is a block diagram showing the principle of a modification of the complex MTI filter according to the present invention. According to the complex MTI filter according to the present invention as shown in FIG. 2, in addition to the elements shown in FIG. 1, there are provided angle accumulating means 5 for receiving angle data to sequentially accumulate angles Δθ itself indicated by the received angle data so as to obtain accumulated angles, and cosine and sine calculating means 6 for obtaining data representative of cos θ The cosine and sine calculating means 6 may be so arranged to obtain cos θ In this manner, according to the present invention, instead of every entry of the rotating angle θ Further, the cosine and sine calculating means 6 may be provided, as shown in FIG. 2, independently of the first and second phase rotating means 1 and 4. Alternatively, it may be so arranged to be incorporated into the first and second phase rotating means 1 and 4. FIG. 3 is a block diagram showing the principle of another modification of the complex MTI filter according to the present invention. According to the complex MTI filter according to the present invention as shown in FIG. 3, there are provided, at the last stage of the complex MTI filter, direct current (DC) elimination filters 7 and 8. There is a possibility that signals output from the quadrature detector 17 (refer to FIG. 13) are superposed on the offset voltage. In a case where this offset voltage is involved in problems, it is preferable to provide the DC elimination filters 7 and 8 as shown in FIG. 3. The complex MTI filter according to the present invention is effective specifically to extract a fluid (e.g. blood flow) information within the human body under examination utilizing a ultrasonic Doppler effect. In this case, applied to the first phase rotating means 1 and the second phase rotating means 4 is angle data being indicative of an angle corresponding to the movement of the clutter within the human body under examination, which movement being measured by utilizing an ultrasonic Doppler effect. FIGS. 4-6 are views useful for understanding an operational principle of the complex MTI filter according to the present invention in conjunction with an ultrasonic Doppler effect. It is not certainly that coordinates depicted in FIGS. 4-6 correspond to each other between the respective drawings. Hereinafter, the explanation will be made referring to the arrangement shown in FIG. 2. It is assumed that data after quadrature detection, on a certain point within the human body under examination, to which ultrasonic pulse beams are emitted eight times to obtain data, is given by (I Hence, the average value Δθ of the angle Δθ The thus obtained average value Δθ is applied to the angle accumulating means 5. The angle accumulating means 5 obtains an accumulating angle θ The cosine and sine calculating means 6 obtains cos θ The first phase rotating means 1 is operative to rotate the respective data (I To change the view point, this indicates that in FIG. 15, the clutter component information 32 shifts on the axis of the Doppler shift frequency f The data passed through the first and second real type MTI filters 2 and 3 are involved in components due to the blood flow excluding components due to the clutter. This indicates that the group A shown in FIG. 5 shifts to a group B. Data of the group B are involved in data wherein blood flow information is extracted, and as exemplarily shown in the group B the phase is rotated on each data. This phase rotating amount corresponds to the blood flow velocity. However, the data of the group B include influence of the frequency shift due to the phase rotation in the first phase rotating means 1 as well as the blood flow information. Thus, it is necessary to perform the frequency shift by the same amount in the opposite direction. Hence, the output data of the first and second real type MTI filters 2 and 3 are passed to the second phase rotating means 4. The second phase rotating means 4 is operative to rotate the respective data by the angle, which is the same as the angle θ The average value Δψ is proportional to the blood velocity V In this way, provision of the respective phase rotating means before and after the two real type MTI filters makes it possible to implement a complex MTI filter in its entirety. And in addition, as will be described later in the preferred embodiments, the phase rotating means can be realized with an extremely smaller scale circuit comparing with the real type MTI filter, and thus iris possible to extremely reduce the circuit scale of the equipment comparing with the conventional complex MTI filter provided with twice real type MTI filters. Further, according to the complex MTI filter of the present invention, it is sufficient to prepare the filter factors by half of that of the conventional complex MTI filter, and thus it is possible to save a ROM for storing the filter factors and the associated wirings, thereby reducing the circuit scale and contributing to the lower cost. While the above description was made with regard to the principle of the complex MTI filter shown in FIG. 2, there may be considered various modifications. For example, instead of the provision of the angle accumulating means 5, it may be arranged such that the rotating angle θ The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings. FIG. 1 is a block diagram showing the principle of the complex MTI filter according to the present invention; FIG. 2 is a block diagram showing the principle of a modification of the complex MTI filter according to the present invention; FIG. 3 is a block diagram showing the principle of another modification of the complex MTI filter according to the present invention; FIG. 4 is a view useful for understanding an operational principle; of the complex MTI filter according to the present invention; FIG. 5 is a view useful for understanding an operational principle of the complex MTI filter according to the present invention; FIG. 6 is a view useful for understanding an operational principle of the complex MTI filter according to the present invention; FIG. 7 is a circuit diagram of the complex MTI filter according to the first embodiment of the present invention; FIG. 8 is a view useful for understanding a principle of the phase rotation of a complex number; FIG. 9 is a block diagram of a factor storage circuit for storing filter factors in the complex MTI filter shown in FIG. 7; FIG. 10 is a circuit diagram of the complex MTI filter according to the second embodiment of the present invention; FIG. 11 is a circuit diagram of the complex MTI filter according to the third embodiment of the present invention; FIG. 12 is a schematic construction view of one example of a conventional ultrasonic diagnostic system; FIG. 13 is a block diagram of one example of a portion equivalent to a blood flow information detecting section 15 as shown in FIG. 12 which is involved in the conventional ultrasonic diagnostic system; FIG. 14 is a block diagram showing an internal construction of a quadrature detector; FIGS. 15(A) and 15(B) are views each showing the characteristic of the MTI filter; FIG. 16 is a block diagram showing a signal processing circuit corresponding to the blood flow information detecting section shown in FIG. 12; FIG. 17 is a view showing the characteristic of a complex MTI filter; FIG. 18 is a circuit block diagram of the conventional real type MTI filter; FIG. 19 is a circuit block diagram of the conventional real type MTI filter; FIG. 20 is a circuit block diagram of a complex MTI filter; and FIG. 21 is a circuit block diagram of a complex MTI filter. Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 7 is a circuit diagram of the complex MTI filter according to the first embodiment of the present invention. An angle accumulating means 100 receives an average angle Δθ of the phase rotation angle Δθ The phase rotating circuit 300 and the phase reversal circuit 500 are the same each other in structure excepting that signs of data to be added in adders are different from each other. FIG. 8 is a view useful for understanding a principle of the phase rotation of a complex number. Assuming that a complex number I+jQ is rotated clockwise by an angle θ, and it is expressed by a complex number I'+jQ', the operation expression is given as follows: ##EQU5## The phase reversal circuit 500 serves to rotate the input data in the opposite direction with respect to the equation (7). In this case, the operation expression is given as follows: ##EQU6## The phase rotating circuit 300 and the phase reversal circuit 500 shown in FIG. 7 perform the operation according to the equations (7) and (8), respectively. A real type MTI filter section 400 is provided with two pieces of eight order of moving average type MTI filters. The arrangement of those two MTI filters is the same as that of the MTI filters shown in FIG. 18, and thus the explanation thereof is omitted. FIG. 9 is a block diagram of a factor storage circuit for storing filter factors n the complex MTI filter shown in FIG. 7. The memory 22 shown in FIG. 16 previously stores, in the form of look-up table, filter factors K0, K1, . . . , K7 to the moving velocity V FIG. 10 is a circuit diagram of the complex MTI filter according to the second embodiment of the present invention. The complex MTI filter according to the second embodiment is different from the complex MTI filter according to the first embodiment shown in FIG. 7 in the point that the moving average type MTI filter in the first embodiment is substituted by the autoregressive type MTI filter. This autoregressive type MTI filter is the same as that shown in FIG. 19. FIG. 11 is a circuit diagram of the complex MTI filter according to the third embodiment of the present invention. According to the complex MTI filter according to the third embodiment, as the real type MTI filter, there is used an MTI filter of the autoregressive moving average type which is a combination of the moving average type and the autoregressive type. According to the complex MTI filter of the present invention, entered complex data is phase-rotated by the corresponding to the moving velocity of the clutter component. Then a predetermined filtering processing is performed by a real type MTI filter assembly, so that clutter component information is eliminated. Thereafter, the output data of the real type MTI filter assembly is reversely phase-rotated. Thus, according to the present invention, it is possible to provide a complex MTI filter which is smaller in a circuit scale and inexpensive. The complex MTI filter of the present invention is specifically provided with the second phase rotating means 4, which is different from the technology disclosed in U.S. Pat. No. 5,170,792. Thus, in a case where the complex MTI filter of the present invention is applied to an ultrasonic diagnostic system, it is possible to detect an absoute blood flow velocity to an ultrasonic probe, but not a relative blood flow velocity to a clutter component velocity. Further, according to the present invention, it is possible to eliminate a DC component at the lower stream stage than the complex MTI filter with respect to a signal flow, thereby providing a high accuracy of ultrasonic diagnostic system. The present invention is not limited to the particular embodiments described above. Various changes and modifications may be made within the spirit and scope of the invention. Patent Citations
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